1. Field of the Invention
The present invention relates to a digital image coding apparatus using watermarking, a decoding apparatus, and coding and decoding methods, and more particularly, to a digital image coding apparatus which allows continuous flow of information by correcting a distorted host image using a signature image when distortion of the host image is severe, a corresponding decoding apparatus, and corresponding encoding and decoding methods.
2. Description of the Related Art
Watermarking is a technology in which an image to be transmitted is transmitted together with an encrypted image for the purpose of providing security and authenticating rights. A receiver side receives the image to be transmitted (hereinafter to be referred to as a host image) together with the encryption image transmitted with confidence (hereinafter to be referred to as a signature image). The signature image is decoded by a special decryption apparatus.
FIG. 1 is a conceptual block diagram illustrating a conventional digital image codec apparatus 1 for generating a watermarked image 8 and extracting a signature image 14 from the watermarked image. In the conventional digital image codec apparatus, during encoding, a DCT (Discrete Cosine Transform) 2 is performed on a host image 3 and a secret signature image 4 to be transmitted, thereby extracting DCT coefficients for the respective images. The DCT coefficients are encoded by an encoder 5. Here, the DCT coefficient components of the signature image 4 are encoded by a separate encryption encoder 6 that controls the encryption operation. In such a manner, the DCT coefficients of the host image 3 and the DCT coefficients of the signature image 4 inserted into the DCT coefficients of the host image 3 after being encoded, can be transmitted. An IDCT (Inverse Discrete Cosine Transform) 7 is performed on the DCT coefficients 2A of the host image and the DCT coefficients 2B of the signature image inserted into the DCT coefficients of the host image, thereby obtaining a watermarked image 8 in which only the host image is visible, while the signature image is not visible.
The signature image 4 watermarked on the host image 3 is decoded by a special decryption apparatus to then be restored. During restoration, IDCT is performed on a watermarked image, thereby extracting DCT coefficients thereof. Next, the extracted DCT coefficients 2A obtained by performing DCT on the original host image 3, are separated from the DCT coefficients of the watermarked image at subtractor 10, thereby extracting DCT coefficient components of the encoded signature image. Then, a decoder 11 decodes the DCT coefficient components of the encoded signature image. Here, the encryption decoder 11 performs a control operation for the decryption process. The DCT coefficients of the signature image decoded by the decryption control operation are again IDCT-performed 13, thereby restoring the signature image.
As described above, in the conventional digital image codec apparatus using watermarking, if it is determined that the current host image is severely distorted, the current image is corrected by a decoding unit using the previous host image. However, if the previous image, rather than the current image, is used in correcting the image, the flow of information becomes discontinuous. Also, when the image is abruptly changed, an error in correcting the image may be generated.
It is an object of the present invention to provide a digital image coding/decoding apparatus and method thereof, which can correct a distorted host image transmitted from a severely noisy environment by using the current image.
To achieve the above object of the present invention, there is provided a digital image coding apparatus including a discrete wavelet transform portion for performing discrete wavelet transform on a host image to be transmitted thereon, thereby outputting Mxc3x97M discrete wavelet coefficients where M is a predetermined positive integer, a significant coefficient extraction portion for extracting a predetermined percentile of upper significant coefficients among Mxc3x97M wavelet coefficients, a pseudo-random number generator for generating pseudo-random numbers in accordance with a predetermined rule corresponding to an encryption code, and a coefficient replacement/combination portion for obtaining replacing location data representing the Nxc3x97N replacing locations, where N is a predetermined positive integer smaller than M, replacing and combining coefficients corresponding to Nxc3x97N replacing locations selected among Mxc3x97M wavelet coefficients with Nxc3x97N significant wavelet coefficients.
Also, the digital image coding apparatus may further include an inverse discrete wavelet transform portion for performing inverse discrete wavelet transform on the replaced Mxc3x97M wavelet coefficients to generate a combined host image, and a compression portion for compressing the combined host image and the encryption code.
Also, the predetermined positive integer N is preferably smaller than or equal to 0.5M.
According to another aspect of the present invention, there is provided a digital image decoding apparatus for decoding bitstreams coded using watermarking, the apparatus including a decompression portion for decompressing the compressed bitstreams to restore a host image and a predetermined encryption code, an image quality evaluation portion for evaluating the quality of the restored host image and generating a control signal that has a first logic state if the quality of the restored host image is less than a predetermined level, and generating a second control signal that has a second logic state if not, a discrete wavelet transform portion for performing discrete wavelet transform on the restored host image in response to the first logic state control signal to obtain Mxc3x97M wavelet coefficients, M being a predetermined positive integer, a pseudo-random number generator for generating pseudo-random numbers in accordance with a rule corresponding to the restored encryption code, a coefficient separating portion for obtaining separating location data representing the Nxc3x97N replacing locations using the pseudo-random numbers, N being another predetermined integer smaller than M, and separating wavelet coefficients of the Nxc3x97N separating locations from Mxc3x97M wavelet coefficients, an inverse discrete wavelet transform portion for performing is inverse discrete wavelet transform on the Nxc3x97N wavelet coefficients to generate a restored signature image, an image scale conversion portion for increasing the scale of the restored signature image into that of the host image, and an image selection portion for selecting a scale-increased signature image in response to the first logic state control signal, and selecting the restored host image in response to the second logic state control signal to output a selected image as a host image.
According to still another aspect of the present invention, there is provided a digital image codec apparatus using watermarking, the apparatus including a first discrete wavelet transform portion for performing a discrete wavelet transform on a host image to be transmitted thereon, thereby outputting Mxc3x97M discrete wavelet coefficients where M is a predetermined positive integer, a significant coefficient extraction portion for extracting a predetermined percentile of upper significant coefficients among Mxc3x97M wavelet coefficients, a first pseudo-random number generator for generating pseudo-random numbers in accordance with a predetermined rule corresponding to an encryption code, a coefficient replacement/combination portion for obtaining replacing location data representing the Nxc3x97N replacing locations, where N is a predetermined positive integer smaller than M, replacing and combining coefficients corresponding to the selected Nxc3x97N replacing locations among Mxc3x97M wavelet coefficients with Nxc3x97N significant wavelet coefficients, a first inverse discrete wavelet transform portion for performing inverse discrete wavelet transform on the replaced Mxc3x97M wavelet coefficients to generate a combined host image, a compression portion for compressing the combined host image and the encryption code, a decompression portion for decompressing the compressed bitstreams to restore the host image and the encryption code, an image quality evaluation portion for evaluating the quality of the restored host image and generating a control signal becoming a first logic state if the quality of the restored host image is less than a predetermined level, and becoming a second logic state if not, a second discrete wavelet transform portion for performing discrete wavelet transform on the restored host image in response to the first logic state control signal to obtain Mxc3x97M wavelet coefficients, M being a predetermined positive integer, a second pseudo-random number generator for generating pseudo-random numbers in accordance with a rule corresponding to the restored encryption code, a coefficient separating portion for obtaining separating location data representing the Nxc3x97N replacing locations using the pseudo-random numbers, N being another predetermined integer smaller than M, and separating Nxc3x97N wavelet coefficients corresponding to the Nxc3x97N separating locations from Mxc3x97M wavelet coefficients, a second inverse discrete wavelet transform portion for performing inverse discrete wavelet transform on the Mxc3x97M wavelet coefficients to generate a restored signature image, an image scale conversion portion for increasing the scale of the restored signature image into that of the host image, and an image selection portion for selecting a scale-increased signature image in response to the first logic state control signal, and selecting the restored host image in response to the second logic state control signal to output a host image.
According to still further another aspect of the present invention, there is provided a digital image coding method using watermarking, the method including the steps of (a) performing a discrete wavelet transform on a host image to be transmitted thereon, thereby outputting Mxc3x97M discrete wavelet coefficients where M is a predetermined positive integer, (b) extracting a predetermined percentile of upper significant coefficients among Mxc3x97M wavelet coefficients, (c) generating pseudo-random numbers in accordance with a rule corresponding to a predetermined encryption code, and (d) obtaining replacing location data representing the replacing locations of Nxc3x97N pixels, where N is a predetermined positive integer smaller than M, and (e) replacing and combining coefficients of Nxc3x97N replacing locations selected among Mxc3x97M wavelet coefficients with Nxc3x97N significant wavelet coefficients.
According to still yet another aspect of the present invention, there is provided a digital image decoding method for decoding bitstreams coded using watermarking, the method including the steps of (a) evaluating the quality of a combined host image and determining whether the quality of the combined host image is greater than or equal to a predetermined level, (b) setting the combined host image as a host image if it is determined in step (a) that the quality of the combined host image is greater than or equal to a predetermined level, and (c) setting as a host image a signature image secretly embedded in the combined host image if it is determined in step (a) that the quality of the combined host image is lower than a predetermined level.
Finally, the invention comprises a program media having stored thereon a computer program embodying the foregoing methods.